Method of producing metal composites which can be processed at high temperatures

ABSTRACT

A method whereby a composite clad metal ingot can be produced for onward processing at higher temperatures which are sufficient to allow the properties of the substrate material to be adequately controlled to meet specified requirements. These higher temperatures are required to avoid the undue levels of work hardening which occurs at the lower temperatures previously employed because of the need to avoid the formation of deleterious intermetallics at the bonded clad substrate interface. Such intermetallics are formed at temperatures above approximately 900° C. and should they be present, will weaken or destroy the bond.

FIELD OF THE INVENTION

This invention relates to the manufacture of clad metal compositesincorporating a steel substrate which can be extended in area by hotrolling at temperatures which are sufficiently high to achieve optimumcontrol of the steel properties, simultaneously preventing the growth ofdeleterious intermetallic products of the interface at the metal layersof the composite and maintaining the anti-corrosive properties of thecladder material.

BACKGROUND TO THE INVENTION

The roll bonding of expensive, corrosion resistant metals to steel orother cheaper substrate materials to form a clad composite material iswell known. Such metal composite materials are used extensively inchemical, petrochemical and similar process plant to minimise the costof such installations. This reduction in cost is achieved by using thecheaper substrate material to provide the composite with the requiredstrength for structural requirements whilst the outer clad ‘veneer’ ofexpensive metal is of a minimum thickness sufficient to provide thenecessary corrosion resistance.

Such composites can be made by a variety of methods, the most frequentlyused being that of roll bonding. This method requires the surfaces ofthe cladder and substrate materials to be cleaned and placed in contactwith each other to form a loosely assembled composite package which isheated to a suitable temperature before passing between rolls whichcompress the parts together and progressively reduce the compositethickness. The resulting deformation causes the cladder and substratecomponents to bond at their interface to form a bonded clad composite ofreduced thickness and extended area.

Not all cladder materials can be roll bonded in this manner because somesuch metals are incompatible for bonding to the chosen substratematerial. Two such examples are titanium and zirconium which cannot beroll bonded directly to a steel or stainless steel substrate because ofthe formation of deleterious intermetallic substances at the interfaceat the temperatures which are required to effect the bond. In suchcases, the otherwise incompatible substrate and cladder components arenormally joined by explosive bonding them together at the final requiredthicknesses. This process is not only relatively expensive but it alsohas technical limitations which restrict the area and thickness ofcomposite which can be produced. To produce comparatively large areas,quality assurance procedures must be highly detailed and rigorouslyenforced by skilled supervision and, despite this, high levels ofrejection of defective components can be experienced which cannot bere-worked. Consequently, expensive material and labour replacement costsare incurred which must be catered for by contingency costing.

Ingots have also been produced by explosive bonding cladder andsubstrate materials of appropriate thickness proportions. These ingotsthen being heated and ‘conversion rolled’ to simultaneously extend thesurface area and reduce the composite to the required overall thickness.

This technique is applicable to many cladder and substrate metalcombinations and has also included titanium or zirconium explosivelybonded to carbon and stainless steel substrates.

The more extensive use of this technique has been inhibited by the facethat both titanium and zirconium, when bonded to these substratematerials, form brittle intermetallics at their bonded interface whenheated to the high optimum temperatures necessary to provide the steelsubstrate with the mechanical properties defined in most pressure vesselspecifications. These intermetallics form at temperatures aboveapproximately, 850° C. and effectively weaken or destroy the bond.Consequently, it is necessary that the conversion rolling of theexplosively bonded ingot be carried out at temperatures below 850° C. toavoid the formation of these intermetallics.

A further complication is that titanium also undergoes a phase change atapproximately the same temperature that the growth of intermetallicoccurs with the normal alpha structure of the titanium transforming tothe beta phase. This has proved a further incentive to maintainconversion rolling temperatures below a temperature of 850° C. Previouswork in these lower temperature ranges, in which the titanium exists inthe alpha condition and is maintained at a temperature below that whereintermetallics are formed, is defined in U.S. Pat. No. 4,612,259. Thispatent discloses the use of two or more interlayer materials to preventthe growth of titanium/steel intermetallics and defines the rollingtemperatures as being below 850° C. and the selected interlayermaterials include materials having melting points lower than the optimumrolling temperatures required to control the steel properties. Suchmaterials will cause the titanium and steel substrate materials toseparate due to the melting of these lower melting point interlayermaterials if they are processed at the higher optimum temperaturesnecessary for adequate control of the steel properties.

Conversion rolling at temperatures of 850° C. and below, hardens thesteel to a greater extent, making compliance with the accepted steelspecifications extremely difficult. In commercial practice, where largeringots are rolled to extended areas, an initial lower ingot temperaturebelow 850° C., results in rapid cooling during the rolling totemperatures which require the reheating of the composite if the rollingis to continue without damaging the rolling mill and, consequently, anycontrol of the steel properties is effectively relinquished. Suchreheating also requires the use of additional furnaces which are,necessarily, extremely large to accommodate the greatly extended areasof the semi-rolled product.

Attempts have been made to control the mechanical properties of thesteel, when rolling at temperatures below 850° C. by modifying thecomposition of the steel. This normally entails a reduction of thecarbon content to levels such that the level of work hardening whichoccurs at these lower temperatures is reduced to acceptable levels. Suchmodified materials, however, are almost invariably unsuitable for theother fabrication procedures which remain to be completed during thefurther construction of the vessels in which the composite isincorporated. Consequently, these techniques have not proved viable as aproduction process.

Attempts have also been made to facilitate the roll bonding of titaniumand zirconium to steel by the use of interlayer materials which arecompatible with both the cladder and substrate components. Suchinterlayers are designed only to effect the roll bonding of thecomponent materials, and are necessarily comparable or below those ofthe cladder and substrate materials. Such techniques have not proved tobe a commercial success and have had only limited market acceptance dueto variable bond quality over the area of the product. The lower meltingpoint of these materials again necessitates their rolling attemperatures below the optimum required for the control of the steelproperties thus limiting their use to applications in which corrosionresistance is the principal acceptance criterion and the mechanicalproperties of the steel are not of overriding importance. The lack ofuniform bonding over the entire area of the clad interface has givensuch products a reputation as having unreliable mechanical properties.

The present invention overcomes the previously described limitations asthe use of appropriate high melting point interlayer materials preventsthe formation of intermetallics between steel and zirconium and betweensteel and titanium despite any phase changes which occur in thematerials at the high rolling temperatures required to produce therequired mechanical properties in the steel substrate. When thecomposite is eventually cooled in the normal manner to temperaturesbelow approximately 850° C., titanium will revert to the original alphaphase condition. In the event that any titanium should remain in thebeta phase form due to subsequent heat treatments required by the steel,for example quenching, this is of no consequence as the corrosionresistance of the titanium will remain unaffected.

SUMMARY OF THE INVENTION

An object of the present invention is to facilitate the conversionrolling of a composite metal ingot of cladder metal and substrate metalin the optimum range of temperatures necessary to provide the substratemetal with the required mechanical properties whilst also ensuring soundbonding of the cladder metal to the substrate metal throughout, andsubsequent to, the processing of the said composite metal ingot.

To this end, the present invention provides a method of producing acomposite material including a layer of metal cladder material and ametal substrate material, wherein an interlayer of metal material isinterposed between said cladder material and said metal substrate, saidinterlayer material being bonded to said metal cladder and said metalsubstrate, said interlayer material being such that upon heating saidcomposite material to temperatures required to hot form the saidcomposite material and provide the said substrate metal with therequired mechanical properties, the bonds between said cladder materialand said interlayer material and between said substrate material andsaid interlayer material, are not adversely affected.

Advantageously, the interlayer material prevents the formation ofintermetallics between the metal cladder and the metal substrate at thetemperatures above 900° C. which are applied, in accordance withconventional practice of the steelmaking art, to provide the substratemetal with the required mechanical properties.

Advantageously, the interlayer material is a metal which does not formintermetallics with a cladder of titanium metal existing in the betaphase form and which is created at temperatures above approximately 850°C.

Preferably the interlayer material is a metal having a melting pointhigher than the metal cladder and metal substrate which does not formintermetallics at the desired rolling temperatures for the substratematerial.

Preferably the interlayer material is explosively bonded to the layer ofcladder metal. It is further preferable that the interlayer is bonded tothe substrate metal.

Preferably the substrate metal is a steel and the cladder metal is acorrosion resistant metal. In one particularly preferred form of theinvention, the substrate metal is a carbon or stainless steel and thecorrosion resistant cladder metal may be titanium or zirconium or alloysof these materials. The interlayer material may be tantalum or niobium(columbium) or alloys of or between these materials.

In a preferred form, the present invention overcomes the aforementioneddifficulties of the prior art by producing an ingot which incorporatesthe higher integrity of an explosive bond and which can be rolled at theoptimum temperatures necessary to produce the required steel propertieswithout adversely affecting the bond between the component materials.More specifically, the interlayer material prevents the formation of theintermetallics between the alpha or beta form of titanium and steel orzirconium and steel throughout the range of temperatures above 900° C.normally employed in the manufacture of steel having the desiredproperties.

In a preferred form of the invention, the bonds between the cladder andinterlayer material and interlayer and substrate metal are manufacturedby explosive bonding the interlayer and substrate materials but theinvention is not limited to this method of fabrication of the bond andother practicable forms of bonding can be applied.

The invention is equally applicable to the process of production of cladpipe or tube by allowing cylindrical clad steel billets or hollows to beconventionally extruded at temperatures similar to those required forhot rolling. Explosively bonded starting stock in the form of billetscan be used to manufacture the clad pipe by conventional manufacturingtechniques without fear of adversely affecting the bond by growth ofintermetallics and simultaneously allowing adequately high temperaturesto be used which facilitate the required metal flow during extrusion andalso produce the desired mechanical properties in the steel substratematerial of the pipe.

In an alternative form of this application, a bond can be made betweenthe cladder material and interlayer material and between the interlayermaterial and substrate material during the course of the extrusionprocess.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a composite of a first embodiment of the present invention.

FIG. 2 shows the composite of FIG. 1 after hot forming.

FIG. 3 shows a composite of a second embodiment of the presentinvention.

FIG. 4 shows the composite of FIG. 3 after hot forming.

FIG. 5 shows a welded composite of a third embodiment of the presentinvention.

FIG. 6 shows the composite of FIG. 3 after hot forming.

DISCUSSION OF PREFERRED EMBODIMENTS

The present invention will be better understood and appreciated from thefollowing discussion of a number of preferred forms the invention maytake.

A first form of the invention will now be described with reference toFIGS. 1 and 2.

A first metal component, which may be carbon steel or stainless steel,forms the cheaper substrate (2) to which is explosively bonded in afirst bonding operation, an interlayer (4) of a higher melting pointmaterial such as tantalum or niobium (columbium). After bonding, acladder material (6) (such as, for example, titanium or zirconium andchosen on the basis of its corrosion resistance in a specificenvironment), is explosively bonded in a second operation to the surfaceof the interlayer material (4) to form a bonded composite (8) ofsubstantial thickness (10) which is then heated to a temperaturesuitable for hot rolling which, in the case of the steel, will be atemperature in the order of 1200° C.

The heated bonded composite (8) is then hot rolled to extend the surfacearea with an associated reduction of the composite thickness (10) toproduce a final composite (12) of the required final thickness (14) asshown in FIG. 2, with the thickness of the substrate (2), interlayer (4)and cladder (6) having been reduced proportionately. The high meltingpoint interlayer material (4) does not form an intermetallic with thesubstrate metal (2) at the high rolling temperature of 1200° C. which istypical of temperatures used in the hot rolling reduction of steel andnor does it form an intermetallic with the cladder material (6) or thebase material (4). Thus the bonded interface (16) between the substrate(2) and the interlayer (4) and the bonded interface (18) between theinterlayer (4) and the cladder (6) remain unaffected by the temperatureand rolling and, thereby, retain their integrity throughout the processof rolling.

An alternative method of producing the bonded composite (8) is to cladthe substrate (2), interlayer (4) and cladder (6) in a single explosivebonding operation by conventional explosive bonding techniques. Theresulting bonded composite (8) is then hot rolled at a temperature of1200° C. in identical fashion to produce a final composite (12) at therequired thickness (14) with the required mechanical properties in thesteel substrate (2).

Another form of the invention will now be described with reference toFIGS. 3, 4, 5 and 6.

A substantially thick ingot of titanium or zirconium (6) is explosivelyclad with a high melting point interlayer material (4) such as tantalumor niobium and a layer of carbon or stainless steel (2) to form a firstcomposite ingot (20), the explosive bonding being achieved either in aone or two stage operation. The outer surface (22) of the steel (2) iscleaned by grinding and a further steel component (24) with a similarlyprepared surface (26) is placed upon the composite ingot (20) with theprepared surfaces (22) and (26) abutting. The interface is sealed with afusion weld (28) and the composite assembly (30) of substantialthickness (32) is then heated to conventional rolling temperatures forsteel and reduced to an appropriate thickness (34) as shown in FIG. 4.During the course of rolling, the two steel surfaces (22) and (26) arejoined by roll bonding to effect a proportional increase in thethickness of the steel element (2 a) which has been substantiallyincreased in area during reduction of the thickness (32) to thethickness (34). This extended area is now sub-sectioned to form a numberof smaller area individual components, one of which (38) is shown inFIG. 5.

The steel surface (22) of this component (38) is prepared by grindingand the composite (38) is placed upon a steel ingot (40) with a suitablyprepared surface (26) and the interface is sealed with a fusion weld(28). This composite assembly is heated to conventional rollingtemperatures and hot rolled to extend the surface area and effect areduction to a final thickness (42) in the final product (44) as shownin FIG. 6, which now consists of a titanium cladding material (6), aninterlayer (4) and steel substrate (2) with the desired proportionalthickness.

During the course of the extensive reduction and heating cycles, thehigh melting point interlayer material (4) prevents the growth ofintermetallics between the titanium (6) and steel (2) components whichwould otherwise occur in the absence of the interlayer (4), at theelevated temperature required to obtain the required properties in thesteel (2), despite the substantial reduction in thickness of theinterlayer (4) to microscopic proportions.

What is claimed is:
 1. A method of producing a composite material,including a layer of metal cladder material and a metal substratematerial wherein an interlayer material is interposed between said metalcladder material and said metal substrate material, which comprisesbonding said metal interlayer material to said metal cladder materialand said metal substrate material, and thereafter hot forming saidcomposite material, said metal interlayer material being such that uponheating said composite material and hot forming at a temperature above900° C., the bonds between said metal cladder material and said metalinterlayer material and between said metal substrate material and saidinterlayer material are not adversely affected, the interlayer materialpreventing the formation of intermetallics and depletion of the bondbetween the substrate material and the cladder material, said metalsubstrate comprising carbon or stainless steel; the metal claddermaterial comprising titanium, alloy thereof, zirconium or alloy thereof;and the metal interlayer material comprising niobium, alloy thereof,tantalum or alloy thereof.
 2. A method as in claim 1 in which the metalinterlayer material has a melting point higher than the metal claddermaterial and the metal substrate material.
 3. A method as in claim 1 inwhich the metal interlayer material is explosively bonded to the layerof metal cladder material and to the metal substrate material.
 4. Amethod as in claim 1 in which the corrosion resistant metal claddermaterial is titanium or an alloy of titanium.
 5. A method in claim 1 inwhich the corrosion resistant metal cladder material is zirconium or analloy of zirconium.
 6. A method as in claim 1 in which the metalinterlayer material is niobium, or an alloy of niobium.
 7. A method asin claim 1 in which the metal interlayer material is tantalum or analloy of tantalum.
 8. A method as in claim 1 wherein the compositematerial comprises layers of said metal substrate, metal cladder andmetal interlayer materials in substantially flat plate or sheet form toprovide a composite flat ingot and said ingot is hot rolled into finalsheet or plate form while avoiding the formation of intermetallicsbetween the metal cladder and metal substrate.
 9. A method as in claim 1wherein the composite material comprises layers of metal substrate,metal cladder and metal interlayer materials of substantiallycylindrical form to provide a composite cylindrical billet and the hotforming comprises hot extruding said composite.